There are a lot of trees in this area and I want to make the player walk behind them. They're all the same tree so I wanted to know if there was a way to copy the walk behind area of one and paste it atop the others.
Easiest way: make them an object (or bunch of objects) instead of a walkbehind.
Second easiest way: draw the walkbehind for one tree in Photoshop (or similar app). You might even be able to magic-wand it directly, depending on the contrast with the background. Make copies of the layer and move them to where the other trees are. Depending on whether they should be one walkbehind or separate ones, give them the same or different colors. Reduce color depth to 256 colors. Flatten. Make sure the palette is arranged in the right order. Import into AGS. (See the manual under "Tutorial|Advanced room features|Importing a file as the walkable area mask" for additional explanation.)
Since I had already made the tree a separate sprite by croping out the rest of the background and coloring everything else white, it was super easy to import the sprite in and turn it into an object. Works like a charm, Cream is behind the tree and everything.
(https://31.media.tumblr.com/50573f28c6e7a91bbb887a7fdf40f456/tumblr_inline_n3onuiRc8X1qmophg.png)
Thanks!